The control of the mass flow of gases is important in many industries. During the manufacture of semiconductors, for example, many of the processes require a precise reaction of two or more gases under carefully controlled conditions. Since chemical reactions occur on a molecular level, the control of mass flow is the most direct way to regulate the gases.
There have been developed in the art a variety of instruments for measuring the mass flow rate of gases from below 5 standard cubic centimeters per minute (SCCM) to more than 500,000 SCCM. The prevalent design of such instruments requires that the flow of the gas be divided into two or more paths. In a typical instrument such as that described in Reissue U.S. Pat. No. Re. 31,570, dated May 1, 1984, and entitled "Fluid Flowmeter", a small flow is routed through a sensor assembly where the mass flow is measured, while most of the flow is routed through a flow splitter section in parallel with the sensor assembly. The sensor assembly contains a capillary tube with two resistance thermometers wound on the outside. The resistance thermometers form two active legs of an electronic bridge; the other two legs are usually fixed resistors. The bridge is carefully designed and manufactured so that the two resistors are as identical as possible in electrical and thermal characteristics. When a voltage is applied across the bridge, current through the resistance thermometers causes them to self-heat. When there is no flow of gas through the capillary tube, both of the thermometers heat up identically. As gas begins to flow through the tube, the gas cools the first resistance thermometer and transfers a portion of that heat to the second one, causing it to get warmer. The temperature difference between the two thermometers is a function of mass flow.
After passing through the sensor and splitter, the gas flows through a valve assembly which precisely controls the mass flow of the gas. In existing valve assemblies, the valve element may be either a ball that cooperates with a conical seat or a flat plate adapted to engage a raised and rounded seat. Balls are preferred because they are typically less expensive, simpler and more precise than flat plates. This is because there is a large supply of low cost standard balls made to a sphericity of 10 millionths of an inch or better on modern ball grinding machines. Also, it is very easy to produce ball seats to similar tolerances using simple coining techniques. Conversely, flat valve elements are specially made and do not benefit from standardization. The raised and rounded seats used for flat valves are difficult to make and even more difficult to repair if physically damaged.
In any case, the valve element and seat must be sealed from impurities in the external atmosphere and this is accomplished by either a flexible diaphragm or bellows welded in place. The gas flow is controlled by an external actuator operable to press on the flexible member to seat the valve element and thereby close the valve. Differential gas pressure or a spring provides the opening force.
In the manufacture of semiconductor devices, and especially those having features of one micron or less, the amount of reactant gas must be carefully controlled and the gas must be completely free from contamination. Particles, vapors and contaminant gases, such as dust, metal, lint, moisture, solvents, oil, air or other process gases can spoil the products. It is therefore important that the flow passages used in mass flow controllers neither trap such contaminants and subsequently release them to the gas stream, nor generate such contaminants during normal calibration and operation.
Typically, friction causes gas valves to deteriorate and generate undesirable small particles which contaminate the reactant gases. One source of such contamination in existing gas valves is the frictional engagement between the ball valve and the walls of the guiding members retaining the ball. Particles thus generated may become trapped in the pocket surrounding the ball and resist removal by purge gases periodically introduced to sweep the gas path. In addition, when valves are used as part of a control system to regulate the flow of gases, friction and particulate matter can cause undesirable hysteresis in the control system. An example of a prior art hermetically sealed gas valve in which these problems may arise is shown in FIGS. 7 and 8 of copending U.S. patent application Ser. No. 07/668,283 filed Mar. 12, 1991 for "Diaphragm Assembly for Transmitting Force and Motion Between Sealed Environments".
It is therefore an overall object of the present invention to provide a valve assembly which minimizes the generation and entrapment of contaminants.
It is a more specific object of the present invention to provide a valve assembly which eliminates unswept volumes in the gas stream which may form contaminant-trapping pockets or recesses.
Still another object of the invention is to provide a valve assembly in which hysteresis due to valve element friction and/or trapped particulate matter is minimized.